Sunday, December 21, 2008

In 3560 switches WRR scheduler is replaced with SRR.SRR is a scheduling service for specifying the rate at which packets are dequeued. With SRR there are two modes, Shaped and Shared (default). Shaped mode is only available on the egress queues. Shaped egress queues reserve a set of port bandwidth and then send evenly spaced packets as per the reservation. In Shared mode, if a higher priority queue is empty, instead of the servicer waiting for that reserved bandwidth to expire, the lower priority queue can take the unused bandwidth. Shared SRR is used when one wants to get the maximum efficiency out of a queuing system, because unused queue slots can be used by queues with excess traffic. Shaped SRR is used when one wants to shape a queue or set a hard limit on how much bandwidth a queue can use. When one uses Shaped SRR one can shape queues within a ports overall shaped rate. SRR shaped mode always take preference over SRR shared weights. The port rate-limit feature ofSRRlimits the total port bandwidth to a percentage of portspeedfrom 10% to 90%.

Configuring SRR shared & shaped dequeing.

sw3(config)#int fastEthernet 0/1

sw3(config-if)#srr-queue bandwidth shape 10 10 0 0

sw3(config-if)#srr-queue bandwidth share 10 20 40 30

Configuring priority queueing in 3560 ports.

sw3(config)#int fastEthernet 0/1

sw3(config-if)#priority-queue out

The weight assigned to the priority queue is simply ignored for SRR calculations. Unlike the 3550, on the 3560, egress priority queue has queue-id 1, not 4.

In 3560 switches four egress queues are there , in that the 1st queue will bethe expedite queue. Each of these queues will have 3 threshold values known as explicit drop thresholds . First 2 explicit drop thresholds are configurable the 3rd threshold represents 100% state / fixed state .

Configuring egress queues

Mapping COS values to 4 queues

In the following example QQueue 1 is configured with 3 thresholds , COS values 2 & 3 are mapped to threshold 1 of queue 1, COS value 5 is mapped to threshold 2 of queue 1 and COS value 0 is mapped to threshold 3 of queue 1 .

In the following example dscpmapping is done for input srr queues DSCP values 0-7 are mapped to queue 1 with a threshold of 1 , DSCP values 8-15are mapped to queue 1 with a threshold of 2 , rest of the DSCP values are mapped to queue 2 with a threshold of 2.